Recently, there has been a renewed interest in the fabrication technology for mixed oxide (MOX) nuclear fuel pellets and, hence, a renewed interest in the sintering operation that comprises the critical processing step in the fabrication of such pellets. Sintering establishes both the physical and chemical properties of the nuclear fuel necessary for reactor irradiation.
Ideally, the residual gas content of sintered fuel pellets should be as low as possible since, as the fuel pellets are heated in the reactor, release of residual gas can cause pressurization of the fuel rods. Fuel designs that can accommodate this released gas require larger plenum volumes or stronger cladding, both of which result in more costly fuel rods. Larger or stronger fuel rods result in a larger reactor core which again increases overall costs.
The sintering of urania fuel has been typically accomplished with a moisture addition to reduce the halide concentrations which carry over from the feed material conversion process. Since very large moisture additions are utilized to assure the removal of halides, moisture can accumulate on the sintered pellet surfaces in the exit portion of the furnace. Humidity in the atmosphere which comes in contact with the sintered pellets can also result in moisture absorption on the pellet surfaces. Urania fuel has been traditionally vacuum outgassed at elevated temperatures to remove all residual moisture. The moisture addition during sintering has unknowingly prevented high residual gas content in urania fuel so that no problem has been identified. The outgassing operation also removes any residual gas from the pellets assuring that high gas content is not a problem.
Since the feed materials for MOX fabrication are very pure (low halide and impurity concentrations), dry atmospheres with no oxygen additions have been utilized to sinter the fuel. This sintering approach tends to prolong the furnace component life because oxygen additions degrade the molybdenum components (eg boats and windings) via the formation of molybdenum oxide. The MOX fuel pellets produced by sintering in a dry atmosphere, however, exhibit residual gas contents that are higher than the values allowed in the fuel design specification. A vacuum outgas process at elevated temperature has been used to reduce the residual gas content to a value below the design limits. The necessity for this step, however, increases the overall cost of the fuel production process.